Mode identification data reducing method for intra-prediction coding

ABSTRACT

A predictive coding system performs predictive encoding by determination of an optimum prediction mode from prediction methods for a pixel signal of a target block. A predicted signal is generated according to the determined mode, and a residual signal is determined. The residual signal and the optimum prediction mode are encoded to generate a compressed signal, which is decoded. The decoded signal is stored as a reconstructed picture sample. During encoding, a candidate prediction mode list is generated that contains elements of optimum prediction modes of previously-reproduced blocks neighboring the target block. A flag indicating whether the list contains an element corresponding to the optimum prediction mode is encoded, and an index to the corresponding element is encoded if the corresponding element is included in the list. The optimum prediction mode can be encoded based on identifying the elements in the list, unless no corresponding element appears on the list.

This application is a continuation of U.S. patent application Ser. No.13/941,235, filed Jul. 12, 2013, which is a continuation ofPCT/JP2011/079071, filed Dec. 15, 2011, which claims the benefit of thefiling date pursuant to 35 U.S.C. § 119(e) of JP2011-004293, filed Jan.12, 2011, all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates a predictive coding system that includesimage predictive encoding and decoding methods, devices, and programsand, more particularly, to methods, devices, and programs for predictiveencoding and decoding using a signal in a frame.

BACKGROUND ART

The compression encoding technologies are used for efficienttransmission and storage of still pictures and video data. The systemsof MPEG1 to MPEG4 and H.261 to H.264 are commonly used for video data.

A predictive coding system provides a method, device, and program forefficient encoding of mode information to identify the intra-frameprediction method of the target block even in the case where a largernumber of prediction modes are provided in the method of generation ofintra-frame predicted signals. to the predictive coding system furtherprovides a method, device, and program for efficient decoding of encodedmode information.

The predictive coding system may perform an image predictive encodingmethod that includes: a region division step of partitioning an inputpicture into a plurality of blocks; a predicted signal generation stepof determining, for pixel signals contained in a target block selectedfrom a plurality of blocks, an optimum prediction mode with the smallestdifference from among a plurality of prediction methods, and generatinga prediction signal according to the optimum prediction signal, andgenerating a predicted signal in accordance with the optimum predictionmode; a residual signal generation step of obtaining a residual signalrepresenting a difference between the pixel signal of the target blockand the predicted signal; a signal encoding step of encoding theresidual signal to generate a compressed signal; a prediction modeencoding step of encoding the optimum prediction mode; and a storagestep of restoring the compressed signal and storing a restored signal asa reconstructed picture sample, or decoded signal, wherein theprediction mode encoding step includes: generating a candidateprediction mode list containing elements of optimum prediction modes ofa plurality of previously-reproduced blocks neighboring the targetblock; encoding a flag to indicate whether the candidate prediction modelist contains an element corresponding to the optimum prediction mode;further encoding an index to the corresponding element in the candidateprediction mode list when there is a corresponding element; when thereis no corresponding element, encoding with a number using the optimumprediction mode, after each element in the candidate prediction modelist is removed.

The predictive coding system may perform an image predictive decodingmethod that includes: an input step of accepting input of compressedpicture data containing a residual signal and encoded information, theresidual signal generated by dividing a picture into a plurality ofblocks and performing predictive encoding of a target block, and theencoded information being about a prediction mode indicative of ageneration method of a predicted signal of the target block; arestoration step of extracting the residual signal of the target blockfrom the compressed picture data to restore a reproduced residualsignal; a prediction mode decoding step of restoring the encodedinformation about the prediction mode to generate an optimum predictionmode; a predicted signal generation step of generating the predictedsignal of the target block, the predicted signal generated based on theoptimum prediction mode; a picture restoration step of adding thepredicted signal to the reproduced residual signal to restore a pixelsignal of the target block; and a storage step of storing the restoredpixel signal as a reconstructed picture sample, or decoded signal,wherein the prediction mode decoding step includes: generating acandidate prediction mode list containing elements of optimum predictionmodes of a plurality of previously-reproduced blocks neighboring thetarget block; decoding a flag to indicate whether the candidateprediction mode list contains an element corresponding to the optimumprediction mode; 1) when the flag indicates that “there is acorresponding element”, further decoding an index that indexes thecandidate prediction mode list and defines an element indicated by theindex as the optimum prediction mode; 2) when the flag indicates that“there is no corresponding element”, further decoding information aboutan REM mode and defining, as the optimum prediction mode, a value of theREM mode, which is converted based on the candidate prediction modelist.

In an embodiment, when the prediction mode information of the targetblock is encoded by performing intra-frame prediction using moreintra-frame prediction modes than in the conventional technology, sincethe candidate prediction mode list consisting of a plurality ofprediction modes is prepared, and an identifier of an element coincidentwith the prediction mode of the target block from the prepared candidateprediction mode list is encoded; the probability that the element iscoincident with the prediction mode of the target block becomes higher,and thus the prediction mode information can be encoded by a smaller bitcount. In other words, there is only one “most probable mode” in theconventional technology, whereas a plurality of “most probable modes”are prepared in the present invention; therefore, the present inventionprovides an effect of increasing the probability of occurrence of a“most probable mode” coincident with the prediction mode of the targetblock.

If the prediction mode of the target block is absent in the candidateprediction mode list, the prediction mode of the target block itself isencoded but, in that case, since a plurality of prediction modes in thecandidate prediction mode list are excluded and new identificationnumbers are assigned to the remaining prediction modes, the predictionmode of the target block can be expressed by a smaller number, allowingencoding with a smaller bit length.

Namely, the predictive coding system provides an effect of enabling moreefficient encoding of the information about the prediction mode in thecase where the intra-frame prediction is carried out by more intra-frameprediction modes than in the conventional technology.

Other systems, methods, features and advantages will be, or will become,apparent to one with skill in the art upon examination of the followingfigures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withinthis description, be within the scope of the invention, and be protectedby the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The predictive coding system, may be better understood with reference tothe following drawings and description. The components in the figuresare not necessarily to scale, emphasis instead being placed uponillustrating the principles of the system. Moreover, in the figures,like referenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a block diagram showing an example of an image predictiveencoding device according to an embodiment of the predictive codingsystem.

FIG. 2 is a schematic diagram showing an example of pixel extrapolationdirections corresponding to intra-frame prediction modes used in animage predictive encoding device according to an embodiment of thepredictive coding system.

FIG. 3 is a flowchart showing example processing of an intra-frameprediction mode encoder according to an embodiment of the predictivecoding system.

FIG. 4 is a schematic diagram showing an example for explaining anencoding process of an intra-frame prediction mode according to anembodiment of the predictive coding system.

FIG. 5 is a flowchart showing an example generation process of REM modenumber (step 360) in the processing of the intra-frame prediction modeencoder (FIG. 3) according to an embodiment of the predictive codingsystem.

FIG. 6 is a schematic diagram for explaining an example of thegeneration process of REM mode number in the processing of theintra-frame prediction mode encoder (FIG. 3) according to an embodimentof the predictive coding system.

FIG. 7 is a block diagram showing example of an image predictivedecoding device according to an embodiment of the predictive codingsystem.

FIG. 8 is a flowchart showing example processing of an intra-frameprediction mode decoder according to an embodiment of the predictivecoding system.

FIG. 9 is a flowchart showing an example of a generation process of aprediction mode of a target block (step 860) in the processing of theintra-frame prediction mode decoder (FIG. 8) according to an embodimentof the predictive coding system.

FIG. 10 is a schematic diagram describing an example process of anintra-frame prediction mode encoding method using two candidateprediction modes, according to an embodiment of the predictive codingsystem.

FIG. 11 is a drawing showing an example hardware configuration of acomputer for executing a program recorded in a recording medium.

FIG. 12 is a perspective view of an example computer for executing aprogram stored in a recording medium.

FIG. 13 is a schematic diagram showing an example of methods ofgenerating a predicted signal of a target block.

FIG. 14 is a schematic diagram in which a plurality of example methodsfor generation of the predicted signal of the target block are broughttogether in a view.

FIG. 15 is a block diagram showing an example of modules of an imagepredictive encoding program.

FIG. 16 is a block diagram showing an example of modules of an imagepredictive decoding program.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below using FIGS.1 to 16.

In encoding systems, a picture which is an encoding target can bedivided into a plurality of blocks and then an encoding/decoding processof the blocks can be carried out. In some examples, to further increaseencoding efficiency, intra-frame predictive encoding can be carried outin such a manner that a predicted signal is generated using aneighboring previously-reproduced pixel signal (restored signal ofcompressed picture data) present in the same frame as a target block,and then a difference signal obtained by subtracting the predictedsignal from a signal of the target block is encoded, such as in theexample of MPEG4 and H.264. In inter-frame predictive encoding,compensation for motion can be made with reference to anotherpreviously-reproduced picture signal present in a frame different fromthat of a target block to generate a predicted signal, and a differencesignal obtained by subtracting the generated predicted signal from asignal of the target block is encoded.

The intra-frame predictive encoding can adopt a method of extrapolatingpreviously-reproduced pixel values neighboring a block as an encodingtarget, in predetermined directions to generate the predicted signal,such as in the example of H.264. FIG. 13 is a schematic diagram examplefor explaining an intra-frame prediction method, such as that used inH.264. In FIG. 13 (A), a block 1302 is a target block and a pixel groupconsisting of pixels A-M (1301) neighboring a boundary of the targetblock is a neighboring region, which is a picture signal previouslyreproduced in past processing. In this case, the predicted signal isgenerated by downwardly duplicating the neighboring pixels (A-D) locatedimmediately above the target block 1302. In FIG. 13 (B), the predictedsignal is generated by rightwardly duplicating previously-reproducedpixels (I-L) located to the left of target block 1304. An example ofspecific methods for generation of the predicted signal are described inU.S. Pat. No. 6,765,964. A difference is calculated between each of ninepredicted signals generated by the example methods shown in FIG. 13 (A)to (I) in this manner, and the pixel signal of the target block, and amethod to provide the smallest difference is defined as an optimumprediction method. These extrapolation methods can be brought togetheras shown in FIG. 14. In FIG. 14, arrows indicate extending directions ofpreviously-reproduced pixels and numbers for the respective directionsare identification numbers of the respective prediction modes. Anidentification number for prediction by an average of surroundingpreviously-reproduced pixels is a predetermined number, such as 2, andis denoted by DC in FIG. 14. These identification numbers are alsoreferred to as identification information about the intra-frameprediction method, or as mode information, or simply as predictionmodes.

The prediction mode of a block undergoing intra-frame prediction can besent to the transmission side. On that occasion, the intra-frameprediction mode of the target block is encoded with reference to theintra-frame prediction modes of an upper neighboring block and a leftneighboring block for the target block. Namely, a comparison is madebetween the intra-frame prediction modes of the upper neighboring blockand the left neighboring block and the block with a smaller value isdetermined as reference mode information (most probable mode). Theintra-frame prediction mode of the target block is encoded based on thisreference mode information. In other examples, other neighboring blocksmay be used.

A symbol to indicate whether the intra-frame prediction mode of thetarget block is identical to the reference mode information can beencoded. When the symbol is, for example, 1, the intra-frame predictionmode of the target block is the same as the reference mode information.When the symbol is different, for example, 0, information about theintra-frame prediction mode of the target block is encoded. However, ifa number indicative of the intra-frame prediction mode of the targetblock is different, such as larger, than a number of the reference modeinformation, encoding is performed after making a modification; such assubtracting one from the number indicative of the intra-frame predictionmode of the target block.

On the reproduction side, the symbol is first decoded in the intra-framepredicted target block. When the symbol is, for example, 1, it is meantthereby that the intra-frame prediction mode of the target block is thesame as the reference mode information. When the symbol is different,for example, 0, the information about the intra-frame prediction mode isfurther decoded. However, if the number of the decoded prediction modeis, for example, equal to or larger than the reference mode information,the intra-frame prediction mode of the target block is determined bymaking the modification in reverse, such as by adding one.

Accuracy of the intra-frame prediction is improved by providing moreintra-frame prediction modes. For example, it is effective to provideoptions of extrapolation for the predicted signal from intermediateangles (directions), in addition to the nine modes shown in FIG. 14.However, the increase of prediction methods can lead to a reduction inencoding efficiency of the identification information (prediction mode)to specify the intra-frame prediction method.

A reason for the encoding efficiency is that the increase in the numberof intra-frame prediction modes results in statistical reduction inprobability of correlation between the prediction mode of the targetblock and the reference mode information (most probable mode). Inaddition, the encoding of the prediction mode itself, in the case ofdisagreement with the reference mode information, can require a largerbit count because the number of intra-frame prediction modes isincreased.

FIG. 1 is a block diagram showing an example of an image predictiveencoding device according to an embodiment of the predictive codingsystem. The image predictive encoding device may be a computing deviceor computer, including for example software, hardware, or a combinationof hardware and software, as described later, capable of performing thedescribed functionality. The image predictive encoding device may be oneor more separate systems or devices included in the predictive codingsystem, or may be combined with other systems or devices within thepredictive coding system. In other examples, fewer or additional blocksmay be used to illustrate the functionality of the image predictiveencoding device. The example image predictive encoding device includesan input terminal unit 101, block divider unit 102, inter-framepredicted signal generation method determiner unit 103, inter-framepredicted signal generator unit 104, intra-frame predicted signalgeneration method determiner unit 105, intra-frame predicted signalgenerator unit 106, changeover switch unit 109, subtracter unit 110,transformer unit 111, quantizer unit 112, de-quantizer unit 113,inverse-transformer unit 114, adder unit 115, frame memory unit 116,intra-frame prediction mode encoder unit 117, entropy encoder unit 118,and output terminal unit 119. Each “unit” described herein, is hardware,or a combination of hardware and software. For example, each unit mayinclude and/or initiate execution of an application specific integratedcircuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit, adigital logic circuit, an analog circuit, a combination of discretecircuits, gates, or any other type of hardware, or combination thereof.Accordingly, as used herein, execution of a “unit” by a processor canalso refer to logic based processing by the unit that is initiateddirectly or indirectly by a processor to complete a process or obtain aresult. Alternatively or in addition, each “unit” can include memoryhardware, such as at least a portion of a memory, for example, thatincludes instructions executable with a processor to implement one ormore of the features of the unit. When any one of the units includesinstructions stored in memory and executable with the processor, theunit may or may not include the processor. In some examples, each unitmay include only memory storing instructions executable with a processorto implement the features of the corresponding unit without the unitincluding any other hardware. Because each unit includes at least somehardware, even when the hardware includes software, each unit may beinterchangeably referred to as a hardware unit, such as the inputterminal hardware unit 101, or the block divider hardware unit 102, forexample.

Below is an example description of the operation of the image predictiveencoding device configured as described above. A signal of a videosequence consisting of a plurality of pictures is fed into the inputterminal 101. A picture as an encoding target is divided, orpartitioned, into a plurality of regions by the block divider 102. In anembodiment of the predictive coding system, each picture is divided, orpartitioned, into blocks, where each block consists of 8×8 pixels, buteach picture may be divided, or partitioned, into blocks of any othersize or shape. Then a predicted signal is generated for a region as anencoding target (hereinafter referred to as “target block”). In anembodiment is the predicted signal can be generated using two types ofprediction methods, inter-frame prediction and intra-frame prediction.

In the inter-frame prediction, a reproduced picture having a differentdisplay time than that of a target picture, and which has been encodedand then restored in the past, or previously decoded, is used as areference picture, and motion information which provides a predictedsignal with the smallest error from the target block is determined fromthe reference picture. Depending upon the situation, it is also possibleto adopt a method of subdividing the target block into small regions anddetermining the inter-frame prediction method for each subdivided smallregion. In this case, the most efficient division method from among avariety of division methods, and corresponding motion information can bedetermined for the entire target block. In an embodiment of thepredictive coding system, this processing can be carried out by theinter-frame predicted signal generation method determiner 103, thetarget block is fed via line L102, and the reference picture is fed viaL119. With regard to the reference picture, a plurality of pictures thathave been encoded and restored in the past (previously decoded), areused as reference pictures. The details of these operations may besimilar to, for example, any one or more of the methods of MPEG-2, 4 andH.264. The motion information and small region division methoddetermined as described above can be fed via line L104 to theinter-frame predicted signal generator 104. These pieces of informationcan also be fed via line L103 to the entropy encoder 118 and are encodedthereby, and the encoded data is output from the output terminal 119.The inter-frame predicted signal generator 104 can acquire referencesignals from the frame memory 116 (via line L119), based on the smallregion division method and the motion information corresponding to eachsmall region, and generates a predicted signal for each small region.The inter-frame predicted signal generated in this manner is sent viaterminal 107 to the next process block.

In the intra-frame prediction, an intra-frame predicted signal isgenerated using previously-reproduced pixel values neighboring a targetblock in the same frame. A generation method of the intra-framepredicted signal is determined by the intra-frame predicted signalgeneration method determiner 105. The processing of the intra-framepredicted signal generation method determiner 105 will be describedlater. Information (prediction mode) about the intra-frame predictionmethod determined in this manner is sent via line L106 to theintra-frame predicted signal generator 106. The information (predictionmode) about the intra-frame prediction method is also sent via line L105to the intra-frame prediction mode encoder 117. The processing of theintra-frame prediction mode encoder 117 will be described later. Theresults of the processing are sent to the entropy encoder 118 to beencoded thereby, and the encoded data is sent from the output terminal119. The intra-frame predicted signal generator 106 acquires neighboringpreviously-reproduced (previously decoded) pixel signals in the sameframe from the frame memory 116 (via line L116), based on theinformation about the prediction method, and generates a predictedsignal by a predetermined method. The intra-frame predicted signalgenerated in this manner is sent via terminal 108 to the next processblock.

From the inter-frame and intra-frame predicted signals obtained asdescribed above, the changeover switch 109 selects the predicted signalwith the smallest error and sends it to the subtracter 110. However,since there is no past picture for the first picture, all target blocksare at first processed by the intra-frame prediction. In this case, theswitch 109 is always connected to the terminal 108 during processing ofthe picture. The intra-frame prediction method and intra-frameprediction mode encoding method described below are also applicable toencoding and decoding of still pictures.

The subtracter 110 subtracts the predicted signal (fed via line L109)from the signal of the target block (fed via line L102) to generate aresidual signal. This residual signal is transformed by a discretecosine transform by the transformer 111 and coefficients thereof arequantized by quantizer 112. Finally, the entropy encoder 118 encodes thequantized transform coefficients and sends the encoded data along withthe information about the prediction method (prediction mode) and otherinformation from the output terminal 119.

For the intra-frame prediction or the inter-frame prediction of asubsequent target block, it may be necessary to perform inverseprocessing and restoration of the compressed signal of the target block.Namely, the de-quantizer 113 can perform de-quantization of thequantized transform coefficients and the inverse-transformer 114 canperform an inverse discrete cosine transform of the transformcoefficients, thereby restoring a residual signal. The adder 115 addsthe restored residual signal to the predicted signal fed through lineL109, to reproduce a picture signal of the target block, which is storedinto the frame memory 116.

The following will describe the intra-frame predicted signal generationmethod determiner 105 used in the predictive coding system. FIG. 2 is aschematic diagram showing pixel extrapolation methods corresponding tointra-frame prediction modes used in an embodiment of the predictivecoding system. In the present embodiment, intra-frame predicted signalsare generated by a total of sixteen methods. Numbers in FIG. 2 areidentification numbers to identify the respective intra-frame predictionmethods and are referred to as prediction mode information or unitprediction modes. In the respective prediction modes (from number 0 tonumber 15), previously-reproduced (or decoded) pixel signals neighboringa target block are extrapolated in directions indicated by respectivearrows in FIG. 2, to generate the intra-frame predicted signals.Specific extrapolation methods about the prediction modes 0 to 8 areshown in FIG. 13 and the calculation methods thereof are described inPatent Literature 1. In each of the prediction modes 9 to 15, similarly,the intra-frame predicted signal is also generated by linearinterpolation from surrounding previously-reproduced pixel signals toduplicate interpolated values in a direction of a corresponding arrow.The present embodiment employs, for example, sixteen intra-frameprediction methods, but it should be noted that the encoding anddecoding methods of prediction mode according to embodiments of thepredictive coding system can also be applied to cases using the othernumbers of prediction modes and other generation methods of predictedsignal.

The intra-frame predicted signal generation method determiner 105generates, a predetermined number, such as sixteen intra-frame predictedsignals, based on the number, such as these sixteen prediction modes,and, for each signal, calculates a difference thereof from the pixelsignal of the target block sent via line L102. It determines aprediction mode which provides the smallest difference, as anintra-frame prediction mode of the target block.

As described above, either the intra-frame prediction or the inter-frameprediction is selected for the target block (by switch 109) and, whenthe intra-frame prediction is selected, the intra-frame prediction modeencoder 117 processes the intra-frame prediction mode of the targetblock. In the encoding method of the intra-frame prediction modeaccording to an embodiment of the predictive coding system, it isnecessary to use the intra-frame prediction modes (identificationnumbers) of previously-encoded blocks, and therefore the intra-frameprediction mode encoder 117 is provided with a storage memory (notshown), for storage of the intra-frame prediction modes (identificationnumbers) of previously-encoded blocks.

FIG. 3 is a flowchart showing the processing of the intra-frameprediction mode encoder 117 according to an embodiment of the predictivecoding system. Step 310 can be first to generate a list of candidateprediction modes. Elements in this list are prediction modes of aplurality of previously-reproduced blocks located around the targetblock. In the present embodiment, prediction modes possessed bysurrounding previously-reproduced blocks 410-450, which are neighboringtarget block 400 shown in FIG. 4 are defined as elements in thecandidate prediction mode list. FIG. 6 (A) is an example of thecandidate prediction mode list and numerical values in respective boxesrepresent identification numbers of the prediction modes correspondingto the respective surrounding blocks (410 to 450). In this example, thesurrounding blocks (410 to 450) have respective prediction modes thatare different from each other, but if the same prediction mode appearsin multiple of the elements, it can be handled as one element. Forexample, if blocks 410 and 420 have the same prediction mode, the numberof elements in the candidate prediction mode list is not 5 but 4. Forexample, the number of elements in the candidate prediction mode listcan be at most 5 and at least 1. If the surrounding blocks neighboringthe target block are “inter-frame” predicted ones, there can be nointra-frame prediction mode. In the present embodiment, mode 2 (DCprediction) is the only element in the candidate prediction mode list.FIG. 6 (A) shows an example of an arrangement of the values of theelements in the candidate prediction mode list in increasing order, butthe candidate prediction mode list may be configured in decreasingorder. In order to construct the candidate prediction mode list forencoding of the prediction mode of the subsequent block, the intra-frameprediction mode encoder 117 stores the prediction mode of the currenttarget block into the aforementioned storage memory.

Next, step 320 is to compare the intra-frame prediction mode of thetarget block with each of the elements in the candidate prediction modelist to check whether there is a coincident element.

When the intra-frame prediction mode of the target block is found in thecandidate prediction mode list, the processing proceeds to step 330. Inthis step, a determined value, such as a “1” is encoded. This “1”indicates that the intra-frame prediction mode of the target block isincluded in the candidate prediction mode list. The next step is toencode an identifier (index) to the element in the candidate predictionmode list coincident with the prediction mode of the target block (step340). In the present example embodiment, 0, 1, 2, 3, and 4 are assignedto respective indices of the boxes from the left in FIG. 6 (A) and, inthe case where the prediction mode of the target block is “8”, 2 isencoded as an index. These indices may be encoded by using base-1 codes(unary codes). For example, codes of (0, 01, 001, 0001, 00001) areassigned to (0, 1, 2, 3, 4), respectively. The last bit in the code ofthe maximum index may be discarded. Thus, for example, the code “00001”for “4” can be “0000”. Another example method is to encode the indicesby fixed-length codes. In that example case, the code length of fixedcodes may be varied depending upon the size of the candidate predictionmode list (the number of elements). For example, in the case where thesize of the candidate prediction mode list is 4, the indices can beencoded by two bits, and in the case where the size is 2, the indicescan be encoded by one bit. Efficient coding is coding of indices basedon the size of the candidate prediction mode list (the number ofelements). In other examples, any other values that include one or morenumbers, letters and/or characters may be used.

When it is determined in step 320 that the intra-frame prediction modeof the target block is absent in the candidate prediction mode list, theprocessing proceeds to step 350. In this step, a predetermined value,such as“0” is encoded. In this example, the “0” indicates that theintra-frame prediction mode of the target block is not included in thecandidate prediction mode list. In this case, it is necessary to encodethe prediction mode of the target block. In the present exampleembodiment the prediction mode of the target block is encoded as “REMmode”. Since it is known that the prediction mode of the target block isabsent in the candidate prediction mode list, an identification value,such as an identification number, to be encoded herein is not theoriginal identification value of the prediction mode, but is instead oneof identification values reassigned to the remaining prediction modesafter exclusion of the elements in the candidate prediction mode list.An illustrative example will be described using FIG. 6. FIG. 6 (A) showsan example of elements in the candidate prediction mode list, which doesnot include an element corresponding to the prediction mode of thetarget block. Therefore, the remaining prediction modes after exclusionof these prediction modes are shown in the example of FIG. 6 (B). Theresult of reassignment of the example values 0, 1, 2, 3, 4 . . . to therespective boxes from the left in this FIG. 6 (B) is shown in theexample of FIG. 6 (C). For example, in the case where the predictionmode of the target block is a value, such as “9”, “9” is not encoded,but a value such as “6” is encoded as REM mode because “6” in FIG. 6 (C)is reassigned to “9” in FIG. 6 (B). The same prediction mode of thetarget block can be encoded by a smaller numerical value, or a smallerbit count. The reassignment of the identification number of theprediction mode in this manner is carried out in step 360 in FIG. 3.

Another execution method of step 360 is shown in the example of FIG. 5.In step 510 the identification number of the intra-frame prediction modefor the determined target block can be used as the REM mode. In step520, the largest element among elements in the candidate prediction listwhich are not yet used in comparison is defined with a predeterminedidentifier, such as X. Step 530 is to compare the REM mode with X. Whenthe REM mode is, for example, larger than X, step 540 is carried out to,in this example, subtract 1 from the value of the REM mode. Step 550 isto check whether there is a not-yet-compared element in the candidateprediction list; if yes, the processing returns to step 520; if no, theprocessing is terminated.

As an example modification of the processing of FIG. 5, step 520 isconfigured to define the smallest element as X and step 530 is changedto “intra-frame prediction mode of target block ≤X?”, with the sameresult. In this case, when the answer of “intra-frame prediction mode oftarget block ≤X?” is no, the processing is immediately terminated.

The value of the REM mode generated in this manner is encoded in step370. In the present example embodiment, the value of the REM mode isencoded by a fixed-length code, but it is also possible to encode thevalue of the REM mode by a variable-length code. The code length ofthese values of the REM mode may be based on the number of elements in acomplementary set of the candidate prediction mode list.

The present example embodiment describes the case where the size S ofthe candidate prediction mode list (the number of elements) was at most5, but S may be an arbitrary number. It is, however, noted that theencoding device and decoding device need to generate this list by thesame method. In the example case where the candidate prediction modelist is generated from the prediction modes of the upper block (420) andthe left block (440) with respect to the target block 400 in FIG. 4,S=2. In this example case, there are two types of candidate predictionmode lists. When the two surrounding blocks both are intra-framepredicted ones, the list contains two elements; when only one of thesurrounding blocks is an intra-frame predicted one, the list containsone element. The case of one list element is shown in FIG. 10 (A) andthe case of two list elements is shown in FIG. 10 (B). In otherexamples, any other blocks may be used.

Node 80 in the example of FIG. 10 (A) indicates whether the candidateprediction mode list contains an element coincident with the predictionmode of the target block. When there is no coincident element, the REMmode is encoded (82). When there is a coincident element (81), there isno need for encoding of an index because the list contains only oneelement. In the example of FIG. 10 (B), similarly, the REM mode isencoded when there is no coincident element in the list (94). When thereis a coincident element (91), there are two elements and therefore anindex to indicate which is coincident between the first and secondcandidates is encoded.

Next, an image predictive decoding method according to the presentinvention will be described. FIG. 7 is an example of a block diagramshowing an image predictive decoding device according to an embodimentof the predictive coding system. The image predictive decoding devicemay be a computing device or computer, including for example software,hardware, or a combination of hardware and software, as described later,capable of performing the described functionality. The image predictivedecoding device may be one or more separate systems or devices includedin the predictive coding system, or may be combined with other systemsor devices within the predictive coding system. In other examples, feweror additional blocks may be used to illustrate the functionality of theimage predictive decoding device. The image predictive decoding devicemay include input terminal unit 700, data analyzer unit 701,de-quantizer unit 702, inverse-transformer unit 703, adder unit 704,predicted signal generator unit 705, frame memory unit 706, intra-frameprediction mode restoration unit 707, and output terminal unit 708.

Describe below is an example of operation of the image predictivedecoding device configured as described above. Compressed data resultingfrom the compression encoding by the foregoing method is input throughthe input terminal 700. This compressed data contains the residualsignal resulting from the prediction and encoding of the target blockobtained by division of a picture into a plurality of blocks, and themode information about the prediction method. The data analyzer 701analyzes the compressed data to extract the residual signal of thetarget block, the information about the prediction method, thequantization parameter, and the motion information in the case of theinter-frame prediction, or encoded information about the aforementionedintra-frame prediction mode for an intra-frame predicted block. Theresidual signal and quantization parameter of the target block are sent(via line L701) to the de-quantizer 702, to be subjected tode-quantization. The result is transformed by an inverse discrete cosinetransform by the inverse-transformer 703.

When the data analyzer 701 determines that the target block is aninter-frame predicted one, the motion information is fed via line L709to the predicted signal generator 705. The predicted signal generator705 acquires a predicted signal from reconstructed pictures in the framememory 706, based on the motion information. On the other hand, when thedata analyzer 701 determines that the target block is an intra-framepredicted one, the mode information about the intra-frame prediction issent via line L710 to the intra-frame prediction mode restoration unit707 and the intra-frame prediction mode is restored and sent to thepredicted signal generator 705. The predicted signal generator 705acquires previously-reproduced (previously decoded) pixel signals in thesame frame from the frame memory 706, based on the intra-frameprediction mode, to generate a predicted signal. Example of generationmethods of intra-frame predicted signals were described above withreference to FIG. 2. The details of the intra-frame prediction moderestoration unit 707 will be described later.

The predicted signal generated by the predicted signal generator 705 issent via line L705 to the adder 704, and the adder 704 adds the restoredresidual signal to the predicted signal to reproduce a pixel signal ofthe target block. The reproduced picture is output via line L704 and, atsubstantially the same time, is stored via line 708 into the framememory 706.

Next, the processing of the intra-frame prediction mode restoration unit707 according to the present example embodiment will be described. Theoutput from the intra-frame prediction mode restoration unit 707 is anidentification value, such as a number, of the intra-frame predictionmode of the target block and is output via line L707 and, atsubstantially the same time, is stored into a memory (not shown) in theintra-frame prediction mode restoration unit 707 because it can be usedfor restoration of the prediction mode of the subsequent block.

FIG. 8 is a flowchart showing an example of the processing of theintra-frame prediction mode decoder according to an embodiment of thepredictive coding system. Step 810 is first to generate a list ofcandidate prediction modes. Elements in this list are prediction modesof a plurality of previously-reproduced blocks (410 to 450) locatedaround a target block, such as target block 400 shown in FIG. 4. Thespecific description is similar to that of step 301 in the example ofFIG. 3. The encoding device and decoding device generate this candidateprediction mode list by the same method.

Next step 820 is to decode at least one bit. When one bit is transmittedvia line L710 from the data analyzer 701, actual decoding processing iscarried out by the data analyzer 701. This one bit can indicate whetherthe intra-frame prediction mode of the target block is included in thecandidate prediction mode list. Then, step 830 is to perform acomparison to determine whether this one bit is a predetermined value,such as “1”. If the one bit is “1”, the processing proceeds to step 840.Otherwise, the processing proceeds to step 850.

Since the intra-frame prediction mode of the target block is included inthe candidate prediction mode list, step 840 is configured to furtherdecode the identifier (index) indicating which element in the candidateprediction mode list coincides with the intra-frame prediction mode ofthe target block. The element in the candidate prediction mode listindicated by the index is the prediction mode of the target block. Forexample, when the index is “2”, the mode identification number “8” inthe third box from the left in FIG. 6 (A) is the prediction mode of thetarget block. In the present embodiment, for example, this index isdecoded as a base-1 code (unary code). As another example, where theencoding used determines the bit length of the index based on the sizeof the candidate prediction mode list (the number of elements), the sizeof the candidate prediction mode list (the number of elements) can besent to the data analyzer 701 (line L711).

Since the intra-frame prediction mode of the target block is notincluded in the candidate prediction mode list, step 850 is configuredto decode the value of the REM mode. In the present embodiment the valueof the REM mode can be restored as a numerical value of a fixed-lengthcode. The value of the REM mode is different from the actualidentification number of the prediction mode (as described withreference to the example of FIG. 5) and therefore, step 860 is to remapthe value to the actual identification number to obtain the intra-frameprediction mode of the target block.

FIG. 9 shows an example of an execution method for returning the a firstmode, such as a REM mode to the actual identification number of theprediction mode. Step 910 is to substitute the decoded value of REM modeinto a second mode, such as a PRED mode. This PRED mode is a variablefor the intra-frame prediction mode of the target block.

Step 920 is to define an element as a predetermined value, such as X,which is the smallest number among elements not used in comparison yetin the candidate prediction mode list. Step 930 is to compare the PREDmode with X. When the PRED mode is larger than or equal to X, step 940is carried out to, for example, add 1 to the value of the PRED mode.Step 950 is to check whether there is a not-yet-compared element in thecandidate prediction list; if yes, the processing returns to step 920;if no, the processing is terminated. The PRED mode after completion ofthis processing provides the actual identification number of theprediction mode of the target block.

Instead of the example processing of FIG. 9, it is also possible toadopt a method of creating the complementary set of the example of FIG.6 (A) as shown in the example of FIG. 6 (B) and defining the (N+1)th(N=the value of REM mode) element from the left, as the prediction modeof the target block.

The intra-frame prediction mode restoration unit 707 is depicted as anindependent function block in FIG. 7, but it may be incorporated intothe data analyzer 701 in other examples. In this case, the line L710 isconnected directly to the predicted signal generator 705 and theintra-frame prediction mode is sent via the line L710 to the predictedsignal generator 705.

The above embodiment describes the encoding of the prediction modeinformation about the intra-frame prediction, but the same encoding anddecoding methods can also be applied to the inter-frame prediction case.The information about the prediction mode in the inter-frame predictioncase may also be encoded and decoded using the candidate prediction modelist. In this case, the candidate prediction mode list contains elementsof information of inter-frame prediction modes of surroundingpreviously-reproduced blocks. Furthermore, the motion information in theinter-frame prediction case can also be similarly encoded and decoded.In this case, the candidate prediction mode list contains elements ofmotion information of surrounding previously-reproduced blocks.

An image predictive encoding program executable by a computer to performthe image predictive encoding of the predictive coding system can bestored in a recording medium or computer readable storage medium.Furthermore, an image predictive decoding program executable by acomputer to perform the image predictive decoding of the predictivecoding system can be stored in a recording medium or computer readablestorage medium. Examples of recording media or computer readable storagemedium include recording media such as flexible disks, CD-ROMs, DVDs, orROMs, or semiconductor memories, or the like.

FIG. 15 is a block diagram showing an example of modules of the imagepredictive encoding program capable of executing at least a portion ofthe image predictive encoding. At least a part of the image predictiveencoding program P100 can be provided with region division module P101,predicted signal generation module P102, residual signal generationmodule P103, signal encoding module P104, prediction mode encodingmodule P105, and storage module P106. Functions implemented uponexecution of the above respective modules by a computer are at leastpart of the functions of the above-described image predictive encodingdevice. FIG. 16 is a block diagram showing an example of modules of theimage predictive decoding program capable of executing at least aportion of the image predictive decoding. At least part of the imagepredictive decoding program P200 can be provided with input module P201,restoration module P202, prediction mode decoding module P203, predictedsignal generation module P204, picture restoration module P205, andstorage module P206. Functions implemented upon execution of the aboverespective modules by a computer are at least part of the functions ofthe above-described image predictive decoding device. The imagepredictive encoding program P100 and the image predictive decodingprogram P200 configured as described above are stored in a recordingmedium or computer readable storage medium, and are executed by acomputer, such as the example described below.

FIG. 11 is a drawing showing an example of a hardware configuration of acomputer for executing a program recorded in a recording medium and FIG.12 a perspective view of an example of a computer for executing aprogram stored in a recording medium. The computer may be included in aDVD player, a set-top box, a cell phone, or any other computing devicethat can include a CPU and is configured or configurable to performprocessing and control based at least partially on executableinstructions.

As shown in the example of FIG. 11, computer 30 is provided with areading device 12 such as a flexible disk drive unit, a CD-ROM driveunit, or a DVD drive unit, a communication port such as a universalserial bus port (USB), Bluetooth port, an infrared communication port,or any other type of communication port that allows communication withan external device, such as another computer or memory device. Thecomputer 30 may also include a working memory 14 that may include anoperating system, a memory 16 that stores data, such as at least part ofprograms, such as programs stored in the recording medium 10. Inaddition, the working memory C14 and/or the memory C16 may include thememory 207 and the memory 306. The working memory C14 and memory C16 maybe one or more non-transitory computer readable storage medium, and caninclude a solid-state memory such as a memory card or other package thathouses one or more non-volatile memories, such as read-only memories.Further, the computer readable medium can include a random access memoryor other volatile re-writable memory. Additionally or alternatively, thecomputer-readable medium can include a magneto-optical or opticalmedium, such as a disk or tapes or any other non-transitory informationstorage medium to capture carrier wave signals such as a signalcommunicated over a transmission medium. A digital file attachment to ane-mail, stored in a storage medium, or other self-contained informationarchive or set of archives may be considered a non-transitorydistribution medium that is a tangible computer readable storage medium.Accordingly, the embodiments are considered to include any one or moreof a computer-readable storage medium or a non-transitory distributionstorage medium and other equivalents and successor information storagemedia, in which data or instructions may be stored. In addition, thecomputer C10 may have a user interface that includes a monitor device 18such as a display, a mouse 20 and a keyboard 22 as input devices, atouch screen display, a microphone for receipt of voice commands, asensor, or any other mechanism or device that allows a user to interfacewith the computer C10. In addition, the computer C10 may include acommunication device 24 for transmission and reception of data andothers, and a central processing unit (CPU) 26, or one or moreprocessors, to control execution of the programs. The processor 26 maybe one or more one or more general processors, digital signalprocessors, application specific integrated circuits, field programmablegate arrays, digital circuits, analog circuits, combinations thereof,and/or other now known or later developed devices for analyzing andprocessing data.

In an example, when the recording medium 10 storing at least part of theprogram P100 is put into the reading device 12, the computer 30 becomesaccessible to the image predictive encoding program P100 stored in therecording medium 10, through the reading device 12, and becomes able tooperate as the previously described image predictive encoding device,based on the image predictive encoding program P100. In an example, whenthe recording medium 10 storing at least part of the image predictivedecoding program P200 is put into the reading device 12, the computer 30becomes accessible to the image predictive decoding program P200 storedin the recording medium 10, through the reading device 12, and becomesable to operate as the previously described image predictive decodingdevice, based on the image predictive decoding program P200.

LIST OF REFERENCE SIGNS

101: input terminal; 102: block divider; 103: inter-frame predictedsignal generation method determiner; 104: inter-frame predicted signalgenerator; 105: intra-frame predicted signal generation methoddeterminer; 106: intra-frame predicted signal generator; 109: changeoverswitch; 110: subtracter; 111: transformer; 112: quantizer; 113:de-quantizer; 114: inverse-transformer; 115: adder; 116: frame memory;117: intra-frame prediction mode encoder; 118: entropy encoder; 119:output terminal; 700: input terminal; 701: data analyzer; 702:de-quantizer; 703: inverse-transformer; 704: adder; 705: predictedsignal generator; 706: frame memory; 707: intra-frame prediction moderestoration unit; 708: output terminal.

What is claimed is:
 1. An image predictive decoding device comprising:input means for accepting input of compressed picture data and encodedinformation, the compressed picture data containing a residual signalgenerated by dividing a picture into a plurality of blocks andperforming predictive encoding of a target block, and the encodedinformation about a prediction mode indicative of a generation method ofa predicted signal of the target block; restoration means for extractingthe residual signal of the target block from the compressed picture datato restore a reproduced residual signal; prediction mode decoding meansfor restoring the encoded information about the prediction mode togenerate the prediction mode; predicted signal generation means forgenerating the predicted signal of the target block based on theprediction mode; picture restoration means for adding the predictedsignal to the reproduced residual signal to restore a pixel signal ofthe target block; and storage means for storing the restored pixelsignal as a reproduced pixel signal, wherein the prediction modedecoding means generates a candidate prediction mode list containingelements of prediction modes of a plurality of previously-reproducedblocks neighboring the target block, and decodes a flag that indicateswhether the candidate prediction mode list contains an elementcorresponding to the prediction mode; when the flag indicates that thecorresponding element is present in the candidate prediction mode list,the prediction mode decoding means further decodes an index indexing thecandidate prediction mode list to obtain an element indicated by theindex as the prediction mode; when the flag indicates that nocorresponding element is present in the candidate prediction mode list,the prediction mode decoding means further decodes a value of a REM(remaining) mode to restore the value of the REM mode as a numericalvalue of a fixed-length code, and remaps the restored value of the REMmode to an identification number of the prediction mode to obtain theintra-frame prediction mode of the target block.
 2. An image predictivedecoding method comprising: an input step of accepting input ofcompressed picture data containing a residual signal and encodedinformation, the residual signal generated by dividing a picture into aplurality of blocks and performing predictive encoding of a targetblock, and the encoded information about a prediction mode indicative ofa generation method of a predicted signal of the target block; arestoration step of extracting the residual signal of the target blockfrom the compressed picture data to restore a reproduced residualsignal; a prediction mode decoding step of restoring the encodedinformation about the prediction mode to generate the prediction mode; apredicted signal generation step of generating the predicted signal ofthe target block based on the prediction mode; a picture restorationstep of adding the predicted signal to the reproduced residual signal torestore a pixel signal of the target block; and a storage step ofstoring the restored pixel signal as a reproduced pixel signal, whereinthe prediction mode decoding step comprises: generating a candidateprediction mode list containing elements of prediction modes of aplurality of previously-reproduced blocks neighboring the target block;decoding a flag that indicates whether the candidate prediction modelist contains an element corresponding to the prediction mode; when theflag indicates that the corresponding element is present in thecandidate prediction mode list, further decoding an index indexing thecandidate prediction mode list to obtain an element indicated by theindex as the prediction mode; when the flag indicates that nocorresponding element is present in the candidate prediction mode list,further decoding a value of a REM (remaining) mode to restore the valueof the REM mode as a numerical value of a fixed-length code, andremapping the restored value of the REM mode to an identification numberof the prediction mode to obtain the intra-frame prediction mode of thetarget block.